Chapter08_Part_I

reelingripehalfSoftware and s/w Development

Dec 14, 2013 (3 years and 7 months ago)

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Chapter 8 (Part I)

Virtual Memory

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Hardware and Control Structures

Memory references are dynamically translated into
physical addresses at run time


A process may be swapped in and out of main memory such
that it occupies different regions

A process may be broken up into pieces that do not
need to located contiguously in main memory

All pieces of a process do not need to be loaded in main
memory during execution

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Execution of a Program

Operating system brings into main memory a few pieces
of the program

Resident set

-

portion of process that is in main memory

An interrupt is generated when an address is needed
that is not in main memory

Operating system places the process in a blocking state

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Execution of a Program

Piece of process that contains the logical address is
brought into main memory


Operating system issues a disk I/O Read request


Another process is dispatched to run while the disk I/O takes
place


An interrupt is issued when disk I/O complete which causes
the operating system to place the affected process in the
Ready state

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Advantages of Breaking up a Process

More processes may be maintained in main memory


Only load in some of the pieces of each process


With so many processes in main memory, it is very likely a
process will be in the Ready state at any particular time

A process may be larger than all of main memory

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Types of Memory

Real memory


Main memory

Virtual memory


Memory on disk


Allows for effective multiprogramming and relieves the user of
tight constraints of main memory

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Thrashing

Swapping out a piece of a process just before that piece
is needed

The processor spends most of its time swapping pieces
rather than executing user instructions

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Principle of Locality

Program and data references within a process tend to
cluster

Only a
few pieces of a process

will be needed over a
short period of time

Possible to make intelligent guesses about
which pieces
will be needed in the future

This suggests that virtual memory may work efficiently

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Support Needed for Virtual Memory

Hardware must support paging and segmentation

Operating system must be able to management the
movement of pages and/or segments between
secondary memory and main memory

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Paging

Each process has its own page table

Each page table entry contains the frame number of the
corresponding page in main memory

A bit is needed to indicate whether the page is in main
memory or not


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Modify Bit in Page Table

Modify bit is needed to indicate if the page has been
altered since it was last loaded into main memory

If no change has been made, the page does not have to
be written to the disk when it needs to be swapped out


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Page Tables

The entire page table may take up too much main
memory

Page tables are also stored in virtual memory

When a process is running, part of its page table is in
main memory

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Inverted Page Table

Used on PowerPC, UltraSPARC, and IA
-
64
architecture

Page number portion of a virtual address is mapped into
a hash value

Hash value points to inverted page table

Fixed proportion of real memory is required for the
tables regardless of the number of processes

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Inverted Page Table

Page number

Process identifier

Control bits

Chain pointer

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Translation Lookaside Buffer

Each virtual memory reference can cause two physical memory
accesses


One to fetch the page table


One to fetch the data

To overcome this problem a high
-
speed cache is set up for page
table entries


Called a
Translation Lookaside Buffer

(TLB)


Contains page table entries that have been most recently used


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Translation Lookaside Buffer

Given a virtual address, processor examines the TLB

If page table entry is present (TLB hit), the frame number is
retrieved and the real address is formed

If page table entry is not found in the TLB (TLB miss), the page
number is used to index the process page table

First checks if page is already in main memory


If not in main memory a page fault is issued

The TLB is updated to include the new page entry


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Translation Lookaside Buffer

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